The present invention relates to signal distortion measurements, and more particularly to a method of measuring instantaneous signal dependent nonlinear distortion in a system under test in response to varying frequency sinusoidal test signals.
Many users need to measure signal amplitude versus frequency of multi-burst and portions of swept sinusoidal test signals in the presence of noise, data compression artifacts and other nonlinear distortions such as frequency aliases common from television set-top boxes and computer graphics cards, quantization error and noise due to a test platform, etc. A multi-burst signal is a signal that has short bursts of sinusoidal signals at increasing, or decreasing, frequencies in a defined time interval, while a linear swept sinusoidal signal is one that linearly increases, or decreases, in frequency in the defined time interval. The variation in amplitude of the bursts or swept signal provide an indication of the frequency response of a system being tested. Presently the signal dependent nonlinear distortion is not measured directly for varying frequency sinusoidal signals in the video domain, but rather is measured using traditional signals, such as ramp or step signals, which often have very low distortion (<1% for example) while the distortion of the sinusoidal signals may be quite large (>50% in some cases). Further confusion in uncorrelated signal fidelity information is due to the fact that sinusoidal signal amplitudes may measure smaller or even larger than the envelope of the signal would indicate on a waveform display due to this otherwise unmeasured distortion.
In distortion testing of sinusoidal signals in video systems the test signals for frequency response include the multi-burst or swept frequency sinusoidal signals. For multi-burst signals typically five or six bursts, each with a different frequency, are used. The specific set of frequencies may vary depending on the video test signal source and format. These frequencies are not always known in advance when making frequency response measurements.
Also for both broadcast digital video and computer video, devices such as set-top boxes and computer graphics cards may reformat the video such that frequencies may shift. These devices also introduce errors such as noise and frequency aliasing. These errors and digital compression artifacts interfere with prior methods for measuring burst frequencies. A recently developed method for measuring magnitude and phase of a given frequency given a point in time or a frequency of interest is disclosed in U.S. Patent Publication Nos. 2004-0161061-A1 and 2004-0161027-A1.
In the area of nonlinear distortion measurements there are several related relevant methods:                nonlinearity measurements of analog video using ramps and steps; and        out of frequency band method including harmonic distortion measurements of sinusoids (especially in audio).The methods as applied to video have been limited to measuring the consistency of ramp slope or step size of corresponding test signals. These methods do not address distortions in sinusoids and for many processes, such as MPEG (Motion Picture Experts Group) compression, the nonlinearities of ramps and steps do not indicate the nonlinearities of sinusoids. Further harmonic distortion measurements that specifically measure magnitudes of harmonics generally do not apply to video because the nonlinear distortions are usually not harmonics, but may be non-harmonic modulation products as in the case of frequency aliases. Also in the method of measuring out-of-band energy, the swept sinusoidal test signal presents a problem in that it contains a wide spectrum of energy. Even if this is time-windowed prior to applying any of this rms out-of-narrowband distortion class of methods, generally noise and signal dependent distortions are not delineated as desired. Further it is desirable to use the swept sinusoid to allow frequency response measurements, with the distortion measurement used as a way to distinguish among linear distortion (amplitude, phase), additive noise (signal independent) and nonlinear distortion. Combining a noise measurement with the out-of-band energy measurement, in order to delineate noise from distortion as a function of the test signal, is problematic for acceptable accuracy levels due to the variance of the noise measurement over a short time interval.        
What is desired is a method for measuring corresponding nonlinear distortion at each point in time or frequency given, specifically an automated method for measuring the distortion of sinusoidal test signal components of impaired analog and digital video that is virtually instantaneous (specific to a point on a video line).